The central pathologic feature of infection with the human immunodeficiency virus (HIV) is dysfunction of the immune system. The precise mechanisms responsible for the immune dysfunction are still poorly understood. A novel hypothesis comes from our recent studies showing that Tat protein from HIV-1 possesses immunosuppressive activity. We found that Tat inhibits antigen-induced lymphocyte proliferation but not mitogen-induced proliferation, a distinction which parallels that seen with lymphocytes from HIV-infected individuals. Preliminary data included herein suggest that Tat may cause immunosuppression by an effect on antigen-presenting cells (APCs). To further characterize the mechanism of the immunosuppressive activity of Tat, we will address the following questions in this proposal. 1) Which cell types that can function as APCs, namely, B cells, monocytes, or dendritic cells, are a target for the immunosuppressive activity of Tat? We will prepare enriched populations of these cells and determine the effects of Tat on their ability to present antigen. 2) Does Tat interfere with the uptake, processing or presentation of antigen? We will measure: a) the ability of Tat to block binding or uptake of radiolabeled antigen, b) the effect of Tat on lymphocyte proliferation induced by peptide antigens, c) the ability of Tat to bind to MHC class II molecules or other "accessory" molecules, and d) the effect of Tat on the mixed lymphocyte reaction. 3) Does Tat alter the production of cytokines that can negatively or positively affect lymphocyte proliferation? We will try to reverse Tat- induced inhibition with supernatants of Con A stimulated PBLs or with specific cytokines. We will also measure production of transforming growth factor beta, an immunosuppressive cytokine. 4) What are the major structure function relationships for the immunosuppressive activity of Tat? If transactivation and immunosuppression show a strong correlation, does the immunosuppressive activity of Tat involve control of the expression of a cellular gene? 5) Does Tat need to be provided extracellularly or would Tat produced internally have the same effect? We will establish a cell line expressing Tat by DNA transfection methods and use the cells as APCs in antigen- induced lymphocyte proliferation assays.